High Power Impulse Magnetron Sputtering (HiPIMS) is a prominent technique to deposit superior materials due to the very energetic growth flux. The origin of this energetic growth flux is believed to be an electric potential structure inside localized ionization zones, the so-called spokes, in the HiPIMS plasma, which rotate in the E × B direction along the racetrack. The measurement of this electric potential or of the electric fields surrounding this ionization zone is extremely challenging due to the very high local power density that obstructs any traditional probe diagnostics. Here, we use a marker technique on the magnetron target to analyze the lateral transport of a target material on a HiPIMS target. We show that the target material is predominantly transported in the E × B direction irrespective of the presence of spokes. However, only when spokes are present, we observe also an enhanced transport in the opposite E × B direction. This is explained by the large electric field at the trailing edges of spokes.
Global models of high-power impulse magnetron sputtering (HiPIMS) plasmas in the literature predict a unique connection between target current waveform and oxidation state of the target (metallic versus poisoned): in the metallic mode, the current waveform reaches a plateau due to metal atom recycling, in the poisoned mode a triangular current waveform is predicted driven by plasma gas recycling. This hypothesis of such a unique connection is tested by measuring the surface chemical composition of chromium magnetron targets directly during reactive high-power impulse magnetron sputtering (r-HiPIMS) by spatially resolved x-ray photoelectron spectroscopy (XPS). The sputtering setup was connected to the ultra-high vacuum XPS spectrometer so that the targets could be transferred between the two chambers without breaking the vacuum. The O2/Ar feed gas ratio, the input power and the pulse frequency of the HiPIMS plasmas were varied. The racetrack oxidation state was measured for different plasma parameters and correlated to the target current waveform shape. It was found that a shift of the target operation from the poisoned mode at low powers to the metallic mode at high powers when operating the discharge at 20 Hz pulse frequency occurs. The transition between these modes was directly correlated with analysis of the Cr2p core level peak on the complete target area. A unique correlation between the metallic and poisoned state of the target and the plateau and triangular current waveform was identified for very low powers and very high powers. In the intermediate power range, such a unique connection is absent. It is argued that the presence of already a small fraction of metal on the target may induce a plateau current waveform despite a significant oxidation of the target. This implies a finite contribution of metal sputtering during the pulse that dominates the recycling and leads to a plateau current waveform. Consequently, the shape of current waveforms cannot easily be connected to target poisoning, but a more detailed modeling of the recycling mechanisms is required.
In-vacuum characterization of magnetron targets after High Power Impulse Magnetron Sputtering (HiPIMS) has been performed by X-ray photoelectron spectroscopy (XPS). Al-Cr composite targets (circular, 50 mm diameter) mounted in two different geometries were investigated: an Al target with a small Cr disk embedded at the racetrack position and a Cr target with a small Al disk embedded at the racetrack position. The HiPIMS discharge and the target surface composition were characterized in parallel for low, intermediate, and high power conditions, thus covering both the Ar-dominated and the metal-dominated HiPIMS regimes. The HiPIMS plasma was investigated using optical emission spectroscopy and fast imaging using a CCD camera; the spatially resolved XPS surface characterization was performed after in-vacuum transfer of the magnetron target to the XPS chamber. This parallel evaluation showed that (i) target redeposition of sputtered species was markedly more effective for Cr atoms than for Al atoms; (ii) oxidation at the target racetrack was observed even though the discharge ran in pure Ar gas without O2 admixture, the oxidation depended on the discharge power and target composition; and (iii) a bright emission spot fixed on top of the inserted Cr disk appeared for high power conditions.
The elementary surface processes occurring on chromium targets exposed to reactive plasmas have been mimicked in beam experiments by using quantified fluxes of Ar ions (400–800 eV) and oxygen atoms and molecules. For this, quartz crystal microbalances were previously coated with Cr thin films by means of high-power pulsed magnetron sputtering. The measured growth and etching rates were fitted by flux balance equations, which provided sputter yields of around 0.05 for the compound phase and a sticking coefficient of O2 of 0.38 on the bare Cr surface. Further fitted parameters were the oxygen implantation efficiency and the density of oxidation sites at the surface. The increase in site density with a factor 4 at early phases of reactive sputtering is identified as a relevant mechanism of Cr oxidation. This ion-enhanced oxygen uptake can be attributed to Cr surface roughening and knock-on implantation of oxygen atoms deeper into the target. This work, besides providing fundamental data to control oxidation state of Cr targets, shows that the extended Berg's model constitutes a robust set of rate equations suitable to describe reactive magnetron sputtering of metals.
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